/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineSessionInterface.h
Called when a user accepts a session invitation. Allows the game code a chance to clean up any existing state before accepting the invite. The invite must be accepted by calling JoinSession() after clean up has completed
typedef TMulticastDelegate_FourParams< void, const bool, const int32, FUniqueNetIdPtr, const [FOnlineSessionSearchResult](API\Plugins\OnlineSubsystem\FOnlineSessionSearchResult) & > FOnSessionUserInviteAccepted